Anthelmintic Resistance

• Parasites usually killed by a specific drug at a specific dose are no longer killed by it. Resistance is heritable in parasite populations.

Key Concepts

• Natural biological consequence of drug treatment that is non-reversible.
• Rate of selection can be reduced to preserve drug efficacy.
• Maintain a population in 'refugia.' 
• Naturally resistant parasites exist at low frequencies due to the role of random mutation.
• Selection enriches for resistant parasites until they dominate (drug pressure). 

Current Resistance

1. Sheep-> haemonchus, teladorsagia, nematodirus, trichostrongylus and fasciola hepatica.
2. Cow-> cooperia, ostertagia, and fasciola hepatica.
3. Horse-> Parascaris Equorum and cyathostomins. 

Rate of selection of a population dictated by:

• Proportion remaining in refugia.
• Frequency of treatment.
• Continuous use of one drug class. 

Factors Promoting Resistance:

• > three treatments/ year. 
• Dose and move to clean pasture.
• Under dose (should dose to the heaviest weight).
• Treat when few larvae on pasture not peak.
• Treat all animals at the same time.

Other Reasons Drugs May Fail:

• Inadequate dosages.
• Decreased drug activity.
• Parasite stage not susceptible.
• Error in Faecal Egg Count/result interpretation.

Maintaining a Population in Refugia:

• Proportion of population not selection for by drug treatment. 
• May be stages on pasture or parasites untreated in a proportion of hosts.
• Provides a pool of sensitive genes to dilute the resistant genes in that population. 

Resistance Diagnosis

• LDA-> larval development assay.
• Faecal Egg Count Reduction Test. 
• FEC-> 10-14 days after anthelmintic treatment. Use large groups 10+. Compare control against drug treated group.
• Effective= 95% reduction compared to controls.
• Equivocal= 90% and above reduction.
• Resistance= less than 90% reductions.

Faecal Egg Counts

• Sometimes used to know when to treat parasites e.g. horse roundworms. 
• The amount of eggs may not be equal to the worm burden e.g. in the instance of cyathostomins. 
• Faecal egg counts on their own can be indicative but are not correlated to the worm burden.